Most modem electronic devices manufactured today contain at least one electrical signal line which is an unwanted source of electrical xe2x80x9cnoisexe2x80x9d, thereby adversely affecting other electronic circuits, both within and external to the electronic device. Generally speaking, this noise exists in the form of electromagnetic interference (EMI) of nearby electrical signals by the offending electrical signal. This EMI may be conducted from the offending electrical signal line to others by way of an electrically conductive path. Alternately, the interference may be radiated from the offending electrical signal line to nearby circuits without the benefit of a directly conductive connection. Oftentimes, the result of such radiated or conducted noise is erroneous or improper operation of the circuit being affected by the EMI, due primarily to unexpected voltage changes in the affected circuit. As a result, protecting electrical circuits from EMI that is generated by other signal lines has long been an important facet of the electronic circuit and device design process.
One example of a source of such noise is a switching power supply or converter, which typically is an electrical circuit designed to convert a power source from one form into another that is usable by another electrical circuit. For example, a direct-current/direct-current (DC/DC) converter transforms an input DC power source, such as a 12 volt (V) DC power source, into an output DC power source with a higher or lower voltage compared to the input source. Other switching power converters, such as AC/DC converters, DC/AC converters, and the like, can exhibit similar noise properties.
One simple example of a DC/DC converter is the buck converter 1 shown in FIG. A. A switch S, which is typically a transistor, is employed to energize an inductor L intermittently via an input DC voltage VIN SO that an output voltage VOUT remains substantially consistent. The inductor L thus is used as an energy-storage component, with the overwhelming majority of that energy then being delivered to a load Zout. The diode D is employed to provide a closed circuit for energy dissipation of the inductor when the switch S is open. The values for the inductor L, a capacitor C, and a resistor R are chosen to restrict certain characteristics of the converter 1 to levels that are acceptable to the load driven. These characteristics include, for example, overshoot and peak-to-peak ripple of the output voltage VOUT.
The opening and closing of the switch S is determined by a switching control circuit 2. The switching control circuit 2 is often comprised in part of an output voltage monitor circuit 3, which monitors the output voltage VOUT of converter 1. The output voltage monitor circuit 3 may consist of, for example, a voltage divider formed by a first and second resistors R1 and R2. The output of the voltage divider is then presented to an input of a first voltage comparator COMP1, which compares that voltage against a DC reference voltage VREF, thus generating an output voltage monitor signal Vovm. A feedback impedance Zf may also be used to control the output of the first comparator COMP1.
Aside from the output voltage monitor circuit 3, the switching control circuit 2 also includes a second comparator COMP2, which compares the output voltage monitor signal Vovm with an oscillating signal Vosc. Often the oscillating signal Vosc is a periodic ramp voltage, although other types of oscillating signals, such as square waves and sinusoidal waves, may also be employed. The output of the second comparator COMP2 thus serves as the switch control signal Vcontrol, operating in pulse-width-modulation (PWM) mode, for opening and closing the switch S based on the demands of the load Zout.
While switching power supplies are well-known for their high efficiency, the typically high current switching levels of the energy storage component, such as the inductor L of the buck converter 1 of FIG. A, normally generate conducted and radiated EMI into surrounding electronic circuits. The power spectral density of this EMI typically takes the form of noise spikes at the fundamental frequency and harmonic frequencies of the PWM control signal used to open and close the switching element of the switching power supply.
Several methods of protecting circuits from EMI generated by switching power supplies have been employed previously. Many such methods involve protecting the sensitive circuits of the electronic device involved from the noise effects of the power converter. For example, the electronic circuit designer often attempts to structure the physical layout of the electronic circuits on a printed circuit board (PCB) so that the generated EMI of the converter will have an attenuated effect on other surrounding circuits. Such efforts include physically routing any offending signals remotely from other sensitive signal lines and circuits, utilizing additional ground planes within the PCB to electrically shield and separate the power converter from surrounding circuits, and the like. Unfortunately, such efforts normally require exorbitant amounts of a PCB designer""s time and effort, and are also error-prone, requiring multiple circuit design revisions in order to reduce sufficiently the effects of the noise on the device.
Other similar solutions involve more substantive circuit additions to shield radiated and conducted noise from circuits that are sensitive to that noise. These additions include the use of large and complex filters on the PCB, chokes, additional metal shielding, shielded cables, and so on.
In contrast to the solutions above, more recent approaches to the problem involve changing the nature of the offending power supply to make that signal less of a noise source to surrounding circuitry. For example, one proposed solution has been to xe2x80x9cditherxe2x80x9d the oscillating signal Vosc by adding a small noise signal to the oscillating signal itself. Dithering of the oscillating signal results in displacing the frequency spectrum of the offending noise a small amount, but does not lower the power level of the frequency spectrum. This solution has been utilized in devices in which other circuits within the device are sensitive to noise at particular frequencies, because the small displacement in the frequency spectrum of the oscillating signal may aid in reducing the effects of the noise on that circuit. However, many electronic devices are susceptible to noise across a wide range of frequencies, making this solution inapplicable in such cases. For example, dithering of the oscillating signal is particularly ineffective for electronic devices such as electronic test and measurement instruments, which often are employed to investigate electronic signals over a very wide band of the frequency spectrum.
Other prior art solutions, such as those indicated in xe2x80x9cCurrent control technique for improving EMC in power converters,xe2x80x9d ELECTRONIC LETTERS, Vol. 37, No. 5, pp. 274-275 (Mar. 1, 2001) by Giral et al., and xe2x80x9cImprovement of power supply EMC by chaos,xe2x80x9d ELECTRONIC LETTERS, Vol. 32, No 12, p. 1045 (Jun. 6, 1996) by Deane et al., focus on the use of chaotic control of DC/DC power converters to reduce the electromagnetic interference normally generated by such circuits. Such solutions succeed in reducing the peaks of the frequency spectrum due to the control signal associated with such converters by spreading out the power of the spectrum at the fundamental and harmonic frequencies. However, such solutions typically do not ensure failsafe operation of the converter being driven by the offending control signal due to its chaotic nature. Adding chaotic control as described by the prior art does not guarantee that the switch will not remain in the closed position, thus potentially causing permanent damage to the inductor of the converter by way of sustained electrical current. By the same token, the circuit described may not prevent excessive periods of time during which the inductor is not being charged, thus allowing the output voltage of the power supply to drop unacceptably.
Another solution, identified by Cahill in U.S. Pat. No. 5,263,055, entitled xe2x80x9cAPPARATUS AND METHOD FOR REDUCING HARMONIC INTERFERENCE GENERERATED BY A CLOCK SIGNALxe2x80x9d, implements a periodic clock signal that is frequency modulated, or alternately, phase modulated, by the output of a pseudorandom noise signal generator. While the power spectral energy of the fundamental and harmonic frequencies of the periodic clock signal is reduced, no control mechanism is present which ensures that the changing frequency of the modulated signal remains within the limits required of the circuit that is being driven by that signal. Hence, such a method, as applied to the control signal of a switching power supply, is also likely to allow the switch associated with the energy storage component of the supply, normally an inductor, to remain open or closed for lengthy periods of time occasionally.
From the foregoing, despite previous attempts to mitigate or reduce EMI generated by switching power supplies, a need still exists for a reliable method of reducing the EMI generated by such supplies. Such a method should both reduce the EMI generated while ensuring that the timing characteristics of the control signal driving the power supply reside within a specified range to ensure effective, nondestructive operation of the supply.
Embodiments of the invention, to be discussed in detail below, provide a switching control circuit for generating a switch control signal for a switching power converter. An output voltage monitor circuit is employed to monitor the output voltage of the power converter, thus producing an output voltage monitor signal. Also, a randomized signal generator is employed to create a randomized signal used as input for a frequency range converter. This range converter, in turn, produces a frequency modulation signal, the current state of which is based on the current state of the randomized signal. Additionally, the frequency range converter limits the current state of the frequency modulation signal so that the oscillating signal that is ultimately produced will operate within the specified frequency range. A variable frequency oscillator then generates the oscillating signal, the frequency of which is based on the current state of a frequency modulation signal. A comparator then compares the voltage of the oscillating signal with the output voltage monitor signal, thereby producing the switch control signal.
By modulating the frequency of the oscillating signal in this manner, the overall EMI produced by the energy storage component of the power converter is reduced in comparison to those power converters that employ oscillating signals of a fixed frequency. Furthermore, by restricting the frequency of the oscillating signal to the specified frequency range, the proper operation of the power converter driven by the oscillating signal is maintained, thus helping to prevent unacceptable voltage dropouts and irreparable damage to the energy storage component.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.